The present invention relates to Optical Time Domain Reflectometry (OTDR).
The measurement principle of OTDR techniques applied in telecommunication today is based on the Rayleigh scatter mechanism. Rayleigh scatter occurs due to refractive index variations in an optical waveguide and forces a part of the optical wave to radiate in all directions. With refractive index changes at distinct locations, caused e.g. by air gaps in optical connectors, typically much higher power levels, compared to backscattered light, are reflected to an OTDR. The low scatter signal levels demand a very sensitive optical receiver, which in case of reflections, with power levels several orders of magnitude higher, is exposed to clipping and saturation. The recovery behavior of the receiver from such an overload is typically given by a delayed and falling decaying curve, which covers the tiny scatter signals for a time frame referred to as deadzone.
GB 2190186 discloses a method for an enhanced spatial detection of optical backscatter by modulating a pseudo random bit sequence onto a light source and transmitting this modulated beam into an optical device.
Rayleigh scattering is not the only mechanism that generates optical return signals. The two other well-known scatter signals are the Brillouin and the Raman scattering. Both occur at wavelengths different from the upstream traveling probe signal. The related technical background is e.g. described, in “Optical Fiber Communications” by J. Senior, Prentice Hall, 1992, pages 91-96.